Magnets for various motors used in vehicles, wind power generation, and the like are demanded to have still greater magnetic properties in order to meet social needs for downsizing and weight saving of electronic devices, and for energy and resource saving to cope with global warming, which has been becoming obvious. Among various measures taken, development of Nd2Fe14B based rare earth sintered magnets having a high magnetic flux density have actively been made. As the applications of the Nd2Fe14B based rare earth sintered magnets are broadened, needs for reduction of the price of the magnets are increasing, and improvement of yield and productivity in magnet production are desired.
A Nd2Fe14B based rare earth sintered magnet is generally prepared by melting and casting a starting material, pulverizing the resulting rare earth magnet alloy into magnet alloy powder, molding the powder in the magnetic field, sintering and ageing the molded product. Pulverization of the rare earth magnet alloy is performed generally by the combination of hydrogen decrepitation effected by subjecting the rare earth magnet alloy to hydrogen absorption/desorption and jet milling effected by bombardment of the rare earth magnet alloy in a jet stream. The rare earth magnet alloy used for the production of a Nd2Fe14B based rare earth sintered magnet contains a Nd2Fe14B based compound phase (sometimes referred to as the 2-14-1 main phase hereinbelow) as the main phase, and an R-rich phase containing more rare earth metal elements than the 2-14-1 main phase (sometimes referred to simply as the R-rich phase hereinbelow). During hydrogen decrepitation, the rare earth magnet alloy is cracked due to the difference in hydrogen absorption rate between the 2-14-1 main phase and the R-rich phase.
As a method for producing a rare earth magnet alloy, Patent Publication 1 discloses a method for casting an alloy having finely-dispersed R-rich phases by rapid cooling and solidifying such as strip casting. This publication also teaches that such a rare earth magnet alloy, having finely-dispersed R-rich phases, has good pulverizability, so that, after sintering, the crystal grains of the 2-14-1 main phase are uniformly coated with the R-rich phases, which provides improved magnetic properties.
Patent Publication 2 discloses that a magnet produced from a rare earth magnet alloy wherein the average distance between R-rich phases is 3 to 12 μm, the value obtained by dividing the standard deviation of the distance between R-rich phases by the average distance between R-rich phases is not more than 0.25, and the volume ratio of the 2-14-1 main phase is not less than 88 vol %, provides improved magnetic remanence, coercivity, and maximum energy product. The publication discloses that this rare earth magnet alloy is obtained by melting a starting material into an alloy melt, supplying the alloy melt onto a roll or a disk to cool and solidify the melt with the average cooling rate until the resulting alloy flakes are separated from the roll or the disk controlled to 50 to 1200° C./sec., cooling the alloy flakes separated from the roll or the disk with the average cooling rate down to a predetermined alloy temperature T+30° C. controlled to not slower than 30° C./sec., and maintaining the alloy flakes in a predetermined temperature range of T±30° C. for 5 to 600 sec.
Patent Publication 3 discloses a method of making a material alloy for an R-T-Q based rare earth magnet including the steps of: preparing a melt of an R-T-Q based rare earth alloy, where R is rare earth elements, T is a transition metal element, Q is at least one element selected from the group consisting of B, C, N, Al, Si, and P, and the rare earth elements R include at least one element RL selected from the group consisting of Nd, Pr, Y, La, Ce, Sm, Eu, Gd, Er, Tm, Yb, and Lu and at least one element RH selected from the group consisting of Dy, Tb, and Ho; cooling the melt of the alloy to a temperature of 700° C. to 1000° C. as first cooling process, thereby making a solidified alloy, maintaining the solidified alloy at a temperature within the range of 700° C. to 900° C. for 15 seconds to 600 seconds; and cooling the solidified alloy to a temperature of 400° C. or less as a second cooling process. This publication also discloses that, in the rare earth magnet alloy obtained by this method, the concentration of the element RH in a portion of the R-rich phase, which is in contact with an interface between the main phase and the R-rich phase, is lower than that of the element RH in a portion of the main phase, which is also in contact with the interface, and crystal grains that form the main phase have minor-axis sizes of 3 μm to 10 μm.
Patent Publication 1: JP-2639609-B
Patent Publication 2: JP-2004-143595-A
Patent Publication 3: WO 2005/105343